That's a tough one. As an Irishman told my father in law once "If I was trying to get there, I wouldn't be starting from here."

The buses down to Porto Vecchio run down the east coast, which probably means you will have to get over to Bastia before you can travel south. Is this to link up with a ferry arrival? Can you take a ferry to somewhere else like Acaccio or Porto Vecchio itself?

The last train from Calvi is 15:45 or 16:21 from L'le Rouse. That will take you as far as Bastia.